Most people born within a sniff of the sea yearn to return to the water. I am one of them, an old duck brought up near the cockles and coracles of the Loughor estuary near Swansea. When I was young, I had little interest in the inlet - it had no crashing waves on its sides, and no ripples heading inland from its big, muddy mouth. To me, it was dull, grey expanse that ran between the ruins of Loughor Castle on one bank, and the chimneys of a factory on the other. It was grimy, it was turgid, it was instantly forgettable.

But on my journey from London to some of Wales's loveliest waters in Pembrokeshire, I am amazed how moved I am - and not only by the view that passes quickly before me, but by the very thought of water itself. I travel over the estuary on a one-carriage train, and suddenly I realise that every body of water grounds the history around it. Water was here before Loughor's castles and factories, after all, and it will be here beyond all of us.

It is this sense of time passing, as well as a growing love of the country in which I was born, that has drawn me back to Welsh waters as I have gotten older. Walking near them reminds us of our mortality, especially when our tightly-laced boots are wobbling on cliff edges. Water is also one of nature's most perfect companions. It has a stillness that helps walking's meditative rhythms. It takes us away from the land on which we wander, and moves us towards its own mysterious depths.

Down in Bosherston, where today's walk begins, these mysteries wallow even deeper. It's not surprising - I am in Arthurian country, a part of Pembrokeshire abundant with myths and legend. There are stories that Excalibur was pulled out of Bosherston's waters, close as they are to both Caldey Island, Wales's craggy contender for the Isle of Avalon, and St Govan's Head, where Sir Gawain is said to be buried. As I approach the ponds through the trees, however, I am unconvinced. It doesn't help that I've read that they were man-made by the damming of several shallow creeks in the 1700s - a millennium after King Arthur's mythical bashing of the oiks.

But that is an irrelevance. I am mainly unconvinced because Bosherston's ponds are made of extraordinary water - clear, pure and glassy - that reveals hundreds of full, purple lilies and tangles of ferns. If there was a magic hand in here or even a sword, you'd spot it in seconds. If you're here in the right season, you'll also find plenty of fish and, if you're especially lucky, a family of otters. Wild garlic, moorhens and nesting swans add life to the pond's flanks, while the bridges between them are also very beautiful. Like the grassy mound that overlooks the ponds on the walk towards Broad Haven, they give walkers an artful perspective on the beauty that surrounds them.

A bracing climb up the dunes leads me to the sea. Broad Haven is a welcoming beach, dotted with ramblers in colourful anoraks. From the headland, I cower at Church Rock, a tiny, jagged island looming up from the sea like a gothic antiquity. Walking on towards Stackpole Head, the cliffs offer gentler treasures. Horses stand in delicate formations of mothers and children, while old, weary sheep watch over their lambs - some of which gambol past me, making me watch my footing more carefully. Far below them, soft waves crest into little caves and tiny, tidal beaches. There is no public access to these from the headland; still, given the perfect circles of pebbles laid out on the sand, rowers must have brought out their boats to these places. I am deeply jealous - these people have had their own secret communion with the sea.

A mile or so later comes Barafundle, the walk's greatest jewel. When the sun shines upon its gently curved shoreline, turning the sea to the colour of duck eggs, it is south Wales's little corner of heaven. The steep walk away from it is forgivable - even more so when the path turns towards Stackpole Quay, and its glorious tearooms. Here, lovely seafood is served, as well as a perfect cup of tea. Joining me from the nearby shores are hoodie-wearing, happy teenagers, clutching fishing rods and their day's catches. Another reason, it seems to me, to advocate the calming wonders of water.

Back at the ponds, thinking of the ways man can mould nature to elegant ends, I hear the distant booming of Castlemartin artillery range, the area's other reminder of history and industry. When these sounds finally stop, and the water keeps on gleaming, I feel very humbled. After all, the water was here before them, and it will be here beyond me.